1. Technical Field
The present invention relates to a glossing device, a fixing device, and an image forming apparatus incorporating the same, and more particularly, to a fixing device that processes a toner image with heat and pressure on a recording medium for imparting gloss, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a fixing device with a glossing capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
Modern image forming apparatuses accommodate a wide range of printing applications with different levels of image quality, using various types of recording media. In particular, printing of photographs and computer-generated images, which typically contain a wide, complete range of visible colors, necessitates a higher level of image quality and uniformity of image gloss than is required for conventional monochrome image formation.
Various techniques have been proposed to meet a growing demand for printers with high-gloss, high-quality imaging performance. Some such techniques employ a special, transparent toner, called “clear toner”, for creating a transparent glossy effect on those areas of a recording medium where no color toner is deposited; others address duplex printing with a uniform, glossy finish on both sides of a recording medium. Among these, several techniques are directed to development of a more sophisticated fixing process.
Structurally, a fixing device with a glossing capability may be constructed of an endless rotary belt on which a recording medium is conveyed while subjected to heat and pressure. The endless belt is looped for rotation around multiple parallel rollers, including a heated roller and a separator roller, with a pressure roller disposed opposite the heated roller via the belt to form a fixing nip therebetween. During operation, a recording medium is conveyed through the fixing nip to process a toner image under heat and pressure. After passage through the fixing nip, the recording medium closely contacts the belt as the belt moves from the heated roller toward the separator roller, and separates from the belt as the belt passes around the separator roller.
For example, a belt-based fixing system has been proposed which includes a thermal pre-fixing unit and a gloss adjustment unit. The pre-fixing unit consists of a pair of opposed heated rollers pressing against each other to form a pre-fixing nip therebetween. The gloss adjustment unit consists of a smooth, endless rotary belt entrained around a pair of motor-driven and idler rollers, with a pressure roller opposite the motor-driven roller to form a main fixing nip therebetween.
In this fixing system, a recording medium is initially passed through the pre-fixing unit, which renders an unfixed powder toner image into a semi-fluid, soft pliable state. After pre-fixing, the recording medium is conveyed to the gloss adjustment unit with the toner image pressed against the endless belt, which imparts gloss to the toner image as the molten toner gradually cools and solidifies while conforming to the smooth surface of the belt. The gloss adjustment unit adjusts glossiness of the toner image by adjusting a distance or duration during which the toner image travels on the belt downstream from the fixing nip.
To date, belt-based fixing devices are designed with a belt cooler for cooling an endless rotary belt during conveyance of a recording medium downstream from a fixing nip, so as to provide uniform cooling and proper separation of the recording medium from the belt after fixing and glossing a toner image thereon.
For example, one known fixing device includes an endless fixing belt entrained around multiple rollers, including a pair of first and second motor-driven rollers, as well as a belt cooler disposed between the first and second rollers inside the loop of the fixing belt.
For maintaining close contact between the fixing belt and the belt cooler, this fixing device is provided with a belt tightening capability, in which the first motor-driven roller is driven at a rotational speed slower than that of the second motor-driven roller, so as to tighten the belt between the first motor-driven roller and the cooler downstream in a longitudinal direction in which the belt rotates around the multiple rollers. Keeping the fixing belt in proper tension prevents the belt from deformation due to being held on the supporting rollers for extended periods of time, as well as sags and creases resulting from thermal expansion/contraction of the belt material subjected to repeated heating and cooling cycles, which would otherwise affect uniformity of gloss across a resulting image.
Although generally successful for its intended purpose, the belt-based fixing device depicted above has several drawbacks. That is, tightening the belt in the longitudinal direction to establish close contact between the belt and the cooler increases the torque required to drive the belt. Also, such arrangement necessitates an extremely high level of precision during assembly of the fixing device. Further, increased longitudinal tension in the belt can lead to accelerated wear and tear of the adjoining surfaces of the belt and the belt cooler, which slide against each other during rotation of the belt.